Cirrus Logic CS5378 - Manual (Page 40)

CS5378
DS639F3 40
12. SINC FILTER
The SINC filter primary purpose is to attenuate out-
of-band noise components from the
ΔΣ modula-
tors. While doing so, they decimate 1-bit
ΔΣ data
into lower frequency 24-bit data suitable for the
FIR and IIR filters.
The SINC filter has three cascaded sections,
SINC1, SINC2, and SINC3, which are each made
up of the smaller stages shown in Figure 23.
The selected output word rate in the FILTCFG reg-
ister automatically determines the coefficients and
decimation ratios selected for the SINC filters.
12.1 SINC1 Filter
The first section is SINC1, a single stage 5th order
fixed decimate by 8 SINC filter. This SINC filter
decimates the incoming 1-bit
ΔΣ bit stream from
the modulators down to a 64 kHz rate.
12.2 SINC2 Filter
The second section is SINC2, a multi-stage, vari-
able order, variable decimation SINC filter. De-
pending on the selected output word rate in the
FILTCFG register, different cascaded SINC2 stag-
es are enabled, as shown in Table 11.
12.3 SINC3 Filter
The last section is SINC3, a flexible multi-stage
variable order, variable decimation SINC filter.
Depending on the selected output word rate in the
FILTCFG register, different SINC3 stages are en-
abled, as shown in Table 11.
12.4 SINC Filter Synchronization
The SINC filter is synchronized to the external sys-
tem by the MSYNC signal, which is generated
from the SYNC input. The MSYNC signal sets a
reference time (time 0) for all filter operations, and
the SINC filter is restarted to phase align with this
reference time.
sinc1
8
5th order 4th order
Figure 23. SINC Filter Block Diagram
1-bit
24-bit
Δ−Σ
2
stage1
sinc2
4th order
2
stage2
sinc2
5th order
2
stage3
sinc2
6th order
2
stage4
sinc2
4th order
5
stage1
sinc3
4th order
5
stage2
sinc3
4th order
5
stage3
sinc3
5th order
2
stage5
sinc3
6th order
3
stage6
sinc3
6th order
2
stage7
sinc3
Output
Input
5th order
5
stage4
sinc3